The invention relates generally to flat panel display manufacturing systems and more particularly to a method of preparing polycrystalline silicon films on flat panel display substrates.
Thin film transistors (TFTs) used in liquid crystal displays (LCDs) or flat panel displays of the active matrix type are fabricated on silicon films deposited on a transparent substrate. The most widely used substrate is glass and amorphous silicon is readily deposited on glass. To provide polycrystalline silicon (alternatively known as polysilicon or p-Si) suitable for TFTs requires crystallization of the as-deposited amorphous silicon. One method of obtaining polycrystalline silicon films on LCD substrates is solid-phase crystallization of deposited amorphous silicon. Solid-phase crystallization is carried out by high-temperature annealing, but glass substrates cannot withstand the temperatures necessary to melt and crystallize silicon. Quartz substrates can withstand high-temperature annealing, but quartz is too expensive for most LCD applications.
Because glass deforms when exposed to temperatures above 600.degree. C., low-temperature crystallization (preferably below 550.degree. C.) is used in solid-phase processing of LCD silicon. The low-temperature process requires long anneal times (at least several hours). Such processing is inefficient and yields polycrystalline silicon TFTs which have relatively low field effect mobility and poor transfer characteristics. Polycrystalline silicon produced by solid phase crystallization of as-deposited amorphous silicon on glass suffers due to small crystal size and a high density of intragrain defects in the crystalline structure.
Excimer laser annealing (ELA) has been actively investigated as an alternative to low-temperature solid-phase crystallization of amorphous silicon film on glass. In excimer laser annealing, a high-energy pulsed laser directs laser radiation at selected regions of the target film, exposing the silicon to very high temperatures for short durations. Typically, each laser pulse covers only a small area (several millimeters on a side) and the substrate or laser is stepped through an exposure pattern of overlapping exposures, as is well known in the art. More powerfull lasers with larger beam profiles are now available or are under active development, reducing the number of exposures required. Regardless of the number and pattern of exposures, excimer laser annealing allows areas of amorphous film to be crystallized without damage to the underlying glass substrate.
The major advantages of excimer laser annealing are the formation of polysilicon grains with excellent structural quality and the ability to process selected areas of a display panel. Polycrystalline silicon produced on transparent substrates by excimer laser annealing have electron mobility characteristics rivaling IC driver circuits currently mounted along the edges of the screen. Thus, it becomes possible to incorporate driver circuitry onto the substrate, simplifying manufacturing.
The most common problem that plagues excimer laser annealing is the narrow process window associated with the development of large and uniform grain sizes. Surface roughness inherent to the process is also troublesome. Research has suggested that improvements in surface conditions, a reduction in defects, and increased crystal size are associated with low oxygen content in ELA polycrystalline silicon films. Oxygen content can be controlled in several ways, the industry standard currently being to perform ELA in a high vacuum (10.sup.-7 Torr) or, somewhat less efficacious, in a rough vacuum (10.sup.-3 Torr). Alternatively, excimer laser annealing has been carried out in chambers filled with non-oxygen ambients such as He, Ar, or Ni with varying results. The association between oxygen content and polycrystalline silicon film quality is still being investigated, but applicant's present invention is directed to a method of controlling oxygen content in a way that is more practical and economical than prior art techniques.
A significant problem with prior art systems for reducing oxygen incorporation into polycrystalline silicon during ELA is the need for a process chamber to house the target substrate. When a process chamber (alternatively called: "chamber," "processing chamber," or "substrate isolation chamber") is used, the beam of the excimer laser must pass into the chamber through a quartz window. Vacuum chambers, in particular, are costly and the quartz windows cost several thousand dollars and have only a limited life, lasting only days or weeks in volume production. Chambers for processing in non-air ambients at near atmospheric pressure are somewhat simpler than vacuum chambers, but sill have costly quartz windows. The costs associated with processing chambers is one reason ELA equipment without substrate isolation chambers are still being manufactured, sold, and used. This despite evidence that ELA performed in air ambients produces polycrystalline silicon with inferior mobility characteristics (and a higher oxygen content) compared with films annealed in vacuum.
It would be advantageous to be able to effectively control the amount of oxygen incorporated in ELA polycrystalline silicon films, keeping the oxygen content below a predetermined threshold, while minimizing the cost of production.
It would also be advantageous to improve the quality of ELA polycrystalline silicon films on flat panel display substrates by performing excimer laser anneals in a predominantly air ambient at atmospheric pressure, eliminating the need for substrate isolation chambers that have costly quartz windows through which the laser beam must pass.
It would be additionally advantageous to improve the quality of ELA polycrystalline silicon films by reducing oxygen incorporation with relatively simple changes to ELA equipment designed for annealing in air ambients.
Accordingly, a method of forming polycrystalline silicon film on substrates used in flat panel displays is provided. The method comprises the steps of first depositing an amorphous silicon film on a substrate used in flat panel displays. The substrate is typically glass. The next step is to excimer laser anneal the amorphous silicon by irradiating one or more target regions of the silicon film with one or more exposures to excimer laser energy. The annealing step transforms the amorphous silicon layer into a polycrystalline silicon layer. The excimer laser annealing step includes providing an ambient environment for the silicon film and substrate which is at atmospheric pressure during the excimer laser anneal. And it further includes displacing the ambient atmosphere from the ambient environment of each target region by directing inert gas onto the surface of the silicon film during irradiation of the target region. In that way, the ambient environment of the silicon film is depleted of oxygen during the excimer laser anneal and the oxygen content of the resultant polycrystalline silicon layer is below a predetermined level.
In the preferred embodiment of the invention, the inert gas used in displacing the ambient atmosphere from the ambient environment of each target region of the silicon film is selected from the group consisting of argon and nitrogen. And among the gases suggested, argon is preferred. The ambient environment which is provided during the excimer laser annealing step is preferably predominantly air and it is the air immediately adjacent the surface of the silicon film which is displaced by the step of directing inert gas onto the surface of the silicon film
It is further suggested that the best method of directing inert gas onto the surface of the silicon film, to displace the ambient (e.g., air) atmosphere, is to direct a continuous flow of inert gas over the surface of the amorphous silicon film. The suggested temperature at which the excimer laser anneal step is performed is room temperature or, alternatively, a temperature of less than 70.degree. C.
The present invention can be used to transform amorphous silicon into polycrystalline silicon on flat panel display substrates (alternatively referred to herein as liquid-crystal display, or LCD, substrates) of any size. Accordingly, the step of irradiating target regions of the silicon film can require exposure of only a single target region to cover the entire substrate or, more commonly, numerous target regions. Since ELA is customarily performed using small cross section laser beams which cover only a fraction of the LCD panel being processed, the irradiating step is typically performed by irradiating successive target regions on the silicon film until the process in complete. Completing the process means either irradiating successive target regions until the entire surface of the silicon film has been excimer laser annealed, or irradiating whichever selected sub-areas or predetermined areas of the film are targeted for conversion to polycrystalline silicon. Yet another alternative variation on the irradiation step is to use a broad cross-section, so-called "single shot" ELA laser which covers the entire surface of the silicon film, in which case only a single target region will be exposed in the process.
The method of the present invention, if carried out in a predominantly air ambient, allows for the displacing of air with inert gas which is directed either at a limited sub-region of the silicon film in the target region of the film where excimer laser annealing is performed, or which floods the entire surface of the silicon film with inert gas during the entire excimer laser annealing process. In the latter alternative, the flow of inert gas is not directed to any specific target area and the entire surface of the silicon film is displaced of air by inert gas while successive target regions are annealed. Regardless of the size of the region wherein air is displaced by inert gas, the purpose of the process is to deplete the ambient environment of oxygen during excimer laser annealing whereby the oxygen content of the resultant polycrystalline silicon layer is maintained below a predetermined level, preferably below 0.5 atomic percent.